This invention relates to fluid sampling and has particular relationship to the sampling of gases, predominantly air, for selected components. Typically, the fluid sampling with which this invention concerns itself is the sampling of air in plants for harmful components pursuant to the Occupational Safety and Health Act (OSHA) or the sampling of the air at a site such as Love Canal contaminated by chemical waste. Sampling of this type is typically carried by a sampler carried or "worn by" the person subjected to the harmful components in the air. The sampling may be carried out throughout the working day of the personnel involved or during measured intervals. On occasion, it is also desirable that the flow of air through the sampler be limited.
A sampler includes a pump which pumps the air through a channel including the collecting or processing device which collects and/or measures the concentration of the harmful components. The processing device is usually interposed in the inlet (suction side) to the pump; it may also be in the outlet (pressure side) particularly in the case of bag collection of chemicals. The components sampled are gases or vapors or solid particulate. The air sampled contains dirt such as dust and other debris and it is necessary that such dirt be filtered out prior to entering the pump. The pump is protected by a so-called pump filter through which the pumped air flows as it is drawn into the channel. The function of the pump filter is to protect the pump from this dirt and debris. The filter becomes loaded with dirt during operation and presents an increasing pressure increment to the pump. In addition, there is an increasing impedance to the flow of pumped air through the channel and the flow or the time rate of the flow of air decreases unless the effect of the filter on the flow is compensated. The flow may also vary for other reasons than accumulation of dirt on the filter. In accordance with the teachings of the prior art, a differential pressure switch is interposed in the outlet of the flow channel to suppress the variation of the air flow. This pressure switch responds to the differential between the pressure in a chamber and the pressure outside of the chamber. The chamber is connected to the region outside through a needle valve. The pressure switch opens and closes repeatedly responsive to the pulsations produced by the pumping. One disadvantage of this expedient is that the repeated operation of the pressure switch has a tendency to cause fatigue in its linkages. Another, and more important, disadvantage is that traces of contaminents such as dirt clog and impede the operation of the needle valve precluding satisfactory, stable and reliable response of the switch to pressure variations in the flow channel. Another drawback is that additional pumping power is required to maintain the pressure required for sensing because an increment of pressure is absorbed for maintaining the control power.
It is an object of this invention to overcome the disadvantages of the prior art and to provide a fluid sampler which shall not include moving parts subject to fatigue and which shall operate reliably and with stability to maintain flow of fluid being sampled substantially constant.